CN109311024B

CN109311024B - Measuring magnetite accumulation in magnetic filters

Info

Publication number: CN109311024B
Application number: CN201680086435.XA
Authority: CN
Inventors: S·唐尼; M·泰勒
Original assignee: Adey Holdings 2008 Ltd
Current assignee: Adey Holdings 2008 Ltd
Priority date: 2016-04-19
Filing date: 2016-06-28
Publication date: 2020-09-22
Anticipated expiration: 2036-06-28
Also published as: DK4005678T3; CA3020041A1; US20230047720A1; EP3646952A1; EP3445496A1; KR20180135938A; EP3445496B1; KR102536046B1; GB2549496A; EP4005678B1; WO2017182764A1; US11484887B2; EP4005678A1; CN109311024A; CA3020041C; US20200324299A1; GB2549496B; PL4005678T3

Abstract

A magnetite level monitoring apparatus (10) for a magnetic filter (100) in a central heating system, the magnetic filter (100) comprising a separation chamber (110), an inlet (112) to the chamber, an outlet (114) to the chamber and a magnetic element (120) disposed within the chamber (110), the magnetic element (120) being for attracting magnetic particles and removing the magnetic particles from water in the system as the water flows through the separation chamber (110), the monitoring apparatus (10) comprising: a housing (14) for being disposed adjacent an exterior of the separation chamber (110); a magnetometer (12) mounted on the housing (14); and a magnetic field guide (18) mounted on the housing (14), the magnetic field guide (14) being disposed between the magnetometer (12) and the exterior of the separation chamber (110) when the housing (14) is mounted on the separation chamber (110); and the output means (24) is adapted to issue a notification when the output of the magnetometer (12) exceeds a preset threshold.

Description

Measuring magnetite accumulation in magnetic filters

Technical Field

The present invention relates to measuring the amount of magnetite captured by a magnetic filter, and in particular to a magnetic filter for use in a central heating system.

Background

It is now common to mount a magnetic filter to the central heating system circuit. In its most basic form, a magnetic filter for a central heating system includes a separation chamber, an inlet to the chamber and an outlet from the chamber, and a magnetic element disposed within the separation chamber for attracting magnetic particles and removing the magnetic particles from the water in the system as the water flows through the separation chamber. An example of a magnetic filter of this type is disclosed in GB 2491246. Magnetic particles are collectively referred to as "magnetite".

The trapped magnetic particles typically settle on the magnetic elements within the filter until a certain time the filter is at full load and no more particles can be trapped. The filter then becomes ineffective until it is cleaned to remove the trapped particulates. The various filters clean in different ways. The most basic method is to disconnect the filter from the central heating system circuit, open the chamber, remove the magnetic element, and clean the magnetite from the magnetic element by scraping or wiping. In most filters, the magnetic element comprises a magnet surrounded by a non-magnetic sleeve. This arrangement makes the cleaning process simpler because the magnetite sticks to the sleeve when the magnet is inside the sleeve, but the magnetite can easily fall off when the magnet is removed. In some filters, the magnet may be removed from the sleeve without opening the chamber. These filters can be cleaned by separating the chamber from the circuit, removing the magnet, opening the drain valve to release the water and flush out the trapped magnetite.

In all known magnetic filters for heating systems, the cleaning step is a step that requires manual intervention in some way. Typically, the installer will recommend that the filter be cleaned regularly, for example once a year, which is also a common maintenance interval for gas boilers.

The problem with setting a specific time period is that: the amount of magnetite actually captured by the magnetic filters varies depending on the installation system in which they are installed and the concentration of corrosion inhibitor in the system. Larger systems with more heat sinks, obviously, may produce larger amounts of magnetite. Heat sinks are also made of different materials, and cast iron heat sinks typically corrode more than steel heat sinks, producing more magnetite. Open systems may suffer from corrosion compared to sealed systems. In addition, a magnetic filter newly installed on an old heating system initially captures a large amount of magnetite, but if the system is properly charged with an inhibitor and sealed to prevent the ingress of oxygen, the magnetite capture rate after filter installation decreases over time.

For these reasons, it is difficult to estimate an appropriate service interval for the magnetic filter. For a completely new system that is sealed and equipped with an anticaking agent, a typical service interval of 12 months may be frequent and unnecessary, but a magnetic filter that has just been installed into the old system may be fully loaded within a month or two.

If the magnetite capture rate actually increases over time, this indicates that there are problems with the heating system, such as cracks causing dilution of the inhibitor and/or air entering the system at some point. It is not uncommon for minor cracks to occur, which the homeowner would not notice, such as slow-seeping joints, where water evaporates directly from the heat pipe structure. In addition, an improperly installed pressure relief valve can result in slow spillage from the external vent tube, which may also go unnoticed.

Such cracks eventually lead to a loss of pressure in the system, at which time the circulation of the heating system must be filled to avoid boiler shutdowns. Any chemical inhibitors are diluted as clean water enters the system water. If this dilution is continued for a long period of time, the inhibitor will become ineffective.

It is clearly preferable to identify this problem at an early stage, but if the magnetic filter is only occasionally opened, it is difficult to identify an increase in the magnetite capture rate. For a magnetic filter that is cleaned by flushing without opening it, it is difficult to estimate the amount of magnetite actually removed with any accuracy. Likewise, there is currently no cost effective method for directly measuring the extent of inhibitor dilution of water in a system.

It is an object of the present invention to provide a device for monitoring the amount of captured magnetite within a magnetic filter.

Disclosure of Invention

According to a first aspect of the invention, there is provided a method of measuring the amount of magnetite captured by a magnetic filter in a central heating system,

the magnetic filter includes a separation chamber, an inlet to the chamber, an outlet to the chamber, and a magnetic element disposed within the chamber for attracting magnetic particles and removing the magnetic particles from the water in the system as the water flows through the separation chamber,

the method comprises the following steps:

providing a magnetometer at a fixed point relative to the magnetic element;

reading the output of the magnetometer; and

if the output from the magnetometer is above a predetermined threshold or below a predetermined threshold, it is indicated on the output device.

The indication may comprise an indication in a simple form, for example by turning on an LED or beeping. In some embodiments, a display or other output device may be provided for indicating when a predetermined level is reached, with more than one predetermined level. For example, in one embodiment, four LEDs are provided that indicate full 25%, full 50%, full 75%, and full 100%. However, the simplest embodiment may have only a single indication that the filter is full or nearly full (e.g., 75% full).

In some embodiments, the output of the magnetometer may be monitored substantially continuously, with an indication being issued whenever the output exceeds or falls below a preset threshold. However, in some embodiments, the output of the magnetometer is read only for a short time, such as at the moment the momentary button is pressed. In a "push to test" system, the output device simply indicates whether the output of the magnetometer is above or below a preset threshold during the test. This may take the simple form of a single LED that lights up when the output is above a threshold and does not light up when the output is below a threshold, or vice versa.

The magnetometer may be arranged outside the separation chamber, e.g. fixed to the wall of the chamber. However, as long as the magnetometer is held in a fixed position relative to the magnetic element, the output of the magnetometer can be used to estimate the amount of magnetite captured on the magnetic element.

The preset threshold may be selected to match the characteristics of the particular magnetic filter to which the method is applied. The device is envisaged to be adjustable or harmonizable to work with different filters. In other words, the predetermined threshold may be variable. It should be noted that even apparently identical filters of the same model may have different magnetic properties, and therefore the preset threshold may need to be specifically selected for each individual filter. However, it is expected that in most cases acceptable results will be obtained by presetting the device with a preset threshold value that is at least applicable to all filters of the same manufacturer and the same model.

During use, as magnetic particles (typically iron and iron oxide "magnetite") accumulate on the magnet within the chamber, the magnetic field strength at the fixed point changes. Due to the typical configuration of the magnetic element (usually north-to-north and south-to-south opposed by multiple magnetic billets), the formation of magnetite is a complex three-dimensional shape, and as magnetite accumulates in some places, the magnetic field strength increases at the chamber walls and decreases elsewhere. Thus, depending on the location of the magnetometer in a particular embodiment, different embodiments may trigger an indication when the magnetometer reading exceeds a threshold, or when the magnetometer reading falls below a threshold. The output of the magnetometer may be positive or negative for the same reason. As magnetite accumulates, it may be positive, increasing or decreasing, or negative.

The magnetometer may be a relative magnetometer. The output of the relative magnetometer is directly proportional to the magnetic field strength, with a fixed but uncalibrated offset. Thus, absolute measurements of the magnetic field strength cannot be obtained directly from the relative magnetometer. In case a relative magnetometer is used, the method may comprise the steps of: measuring the output of the magnetometer when the filter is free of magnetite; and calculating a preset threshold value by adding a fixed offset to the measured "null" value. To facilitate this, the method may comprise the steps of: an indication is received from the input device that the filter is empty, possibly because the filter is new or because it has just been cleaned. It should be understood that the offset may be positive or negative.

Alternatively, the magnetometer may be an absolute magnetometer.

The magnetometer may be any device suitable for measuring magnetic forces, which term includes, for example, hall effect sensors and coils.

In a simple embodiment, the input means for indicating that the filter is empty may be a simple button.

It should not be strictly necessary to "recalibrate" the magnetometer each time the filter is cleaned by indicating that the filter is empty, although if the magnetometer is recalibrated when cleaned, this should take into account any magnetic changes in the magnetic elements in the filter, which changes occur over time and which changes occur regardless of the type of magnetometer used.

A magnetic field guide may be provided. The magnetic field guide may be disposed between the magnetometer and the magnetic element, for example between the magnetometer and the chamber, wherein the magnetometer is located outside the chamber. Alternatively, the magnetic field guide may be disposed in other locations, such as around or behind the magnetometer (i.e., the magnetometer is disposed between the magnetic field guide and the magnetic element). The magnetic field guides direct the magnetic field lines to the area where the magnetometer is located (which may be outside the separation chamber). Preferably, the magnetic field guiding means is made of a material having a low magnetic reluctance but being difficult to permanently magnetize. Grade 430 stainless steel is known to be an advantageous material for manufacturing the magnetic field guide.

In some embodiments, multiple components disposed at different locations may be provided to form a magnetic field guide for guiding magnetic field lines.

The purpose of the magnetic field guide is to compensate for changes in the direction of the magnetic field from the magnetic element within the separation chamber. Many magnetic filters use a magnetic element formed from a stack of substantially cylindrical permanent magnet blanks magnetized such that north and south poles are nominally located at either flat end of the cylinder. The magnetic blanks are located in a stack adjacent north to north and south to south of each other. However, it is known that it is difficult to obtain an economical supply of blanks, which are magnetized along a line through the central axis of the blank. Many magnetic filters comprise a blank whose magnetic axis is tilted up to 20 ° with respect to its physical axis. This does not significantly affect the effect of the magnetic filter in attracting and retaining magnetite, but it is certainly difficult to reliably interpret the output of the magnetometer to determine how much magnetite is retained in the filter.

The magnetic field guide effectively pulls the magnetic field lines to a known point, making the correlation between the output of the magnetometer and the amount of magnetite captured by the magnetometer more consistent for a series of magnets with different characteristics.

The method may further comprise the steps of sampling and recording magnetometer readings, and determining a rate of change from the recorded readings. The rate of change is related to the growth rate of magnetite on the magnet within the filter. It should be understood that an increase in the rate of change may mean an increase in a rate of change that is gradually increasing in negative values or gradually decreasing in negative values, as well as an increase in a rate of change that is gradually increasing in positive values or gradually decreasing in negative values. The reading from the magnetometer is either an increasing or decreasing negative value or an increasing or decreasing positive value, depending only on the position and orientation of the magnetometer relative to the magnetic element. The growth rate of magnetite is generally expected to remain reasonably constant or decrease slightly, as oxygen dissolved in the water of the system is depleted, and when a filter is installed, any magnetite is captured and removed once it is present in the system. Thus, an increased growth rate may be an indication of a problem, such as the presence of a crack in the system or the failure of the system to administer the correct dose level of inhibitor. If the increase in the growth rate is above a certain threshold, an indication may be issued on the output device. In a simple embodiment, the increased growth rate can be identified by monitoring the time it takes to fill the filter after each cleaning of the filter, the time it takes for the magnetometer reading to reach a preset threshold. The "need for service" interval reduction indication may be problematic.

The method may further comprise the steps of: providing a pressure sensing device to measure the static pressure within the separation chamber; sampling and recording the static pressure measurement; and identifying a pressure increase event from the recorded data. A pressure increase event may be defined as an increase in system pressure beyond a predetermined amount in less than a predetermined period of time. For example, an increase of more than 0.5bar in less than 3 minutes. A pressure increase event may indicate that water has been added to the heating system through the fill loop.

An indication may be provided on the output device after a predetermined number of pressure increase events have occurred. This indication provides an alert that water has been added to the system, which may dilute the amount of inhibitor in the water of the system. Alternatively or additionally, the indication may be based on a combination of magnetite growth data and pressure sensing data. The combination of these two data sources can be used to reasonably accurately estimate the dilution level of the inhibitor in the system.

Where a pressure sensor is provided, an alarm may be raised when the pressure is below a predetermined threshold. The threshold may be set higher than the pressure normally at which the boiler is shut down, but lower than the normal operating pressure of the system.

In a simple embodiment, the indication from the output device may be a simple visual or audible output, such as turning on an LED or activating a sound generator. However, in some embodiments, the indication may be sent via a wired or wireless text or data communication means, such as a GSM module for sending SMS text messages, or a GSM module for sending data packets. Further, where communication means are provided, an input indication that the filter has been cleaned may be received by way of the data communication means.

Examples of suitable data communication means include GSM modules, bluetooth (RTM) modules, bluetooth low energy modules, "ZWave' modules", "Zigbee" modules, low power radio modules and WiFi modules. Various other wireless communication means will be within the knowledge of those skilled in the art. In some embodiments, a wired communication device may also be suitable. The choice of communication device will depend on factors such as available power supply, public mobile telephone network coverage at the installation site, etc.

According to a second aspect of the invention, there is provided a method of measuring the amount of magnetite in a magnetic filter in a central heating system.

The magnetic filter includes a separation chamber, an inlet to the chamber, an outlet to the chamber, and a magnetic element disposed within the chamber for attracting magnetic particles and removing the magnetic particles from the water in the system as the magnetic particles flow through the chamber,

the measuring device includes:

a housing placed at a fixed point relative to the magnetic element;

a magnetometer mounted on the housing; and

an output device adapted to indicate whether the output from the magnetometer is above or below a preset threshold.

Preferred/optional features of the second aspect of the invention are set out in claims 17 to 31.

Drawings

For a better understanding of the present invention and to show more clearly how it may be carried into effect, an embodiment will now be described, by way of example, with reference to the accompanying drawings, in which:

FIG. 1 shows an exploded perspective view of a monitoring device according to the present invention having a magnetic filter;

FIG. 2 is a graph of magnetometer output as the magnetometer is outside the filter at various locations for filters containing different magnetite content;

FIG. 3 shows a graph of magnetometer output as a function of magnetite content in a filter for a magnetometer positioned at a fixed location outside the filter;

FIG. 4 is a graph from a computer simulation showing the effect of a magnetic field guide; and

FIG. 5 is a perspective view of a test apparatus in an alternative embodiment of the invention.

Detailed Description

Referring first to FIG. 1, a magnetite level monitoring device, generally designated 10, has a magnetic filter, generally designated 100. The magnetic filter comprises a separation chamber 110, an inlet 112 of the separation chamber 110 and an outlet 114 of the separation chamber 110. A magnet (not visible in the figures) is disposed within the chamber, extending substantially centrally along the longitudinal axis a-a of the chamber. During use, the inlet 112 and outlet 114 are connected to a central heating system loop such that water flows through the separation chamber 110. Any magnetic particles suspended in the system water are attracted to and held on the magnets and remain in the separation chamber until the magnets are cleaned.

The monitoring device 10 comprises a magnetometer 12. In this embodiment, the magnetometer is provided in the form of a surface mountable integrated circuit that is soldered to the printed circuit board 14. In this embodiment, a honeywell (rtm) HMC5883L 3 axis digital compass IC is used. The printed circuit board 14 also mounts and provides connections to other electronic devices, which will not be described in detail, but which will be familiar to those skilled in the art.

The PCB 14 is provided with a hole 16 at each of its four corners. The holes 16 correspond to mounts 116 located outside the separator chamber 110 so that screws can be used to securely attach and hold the PCB 14 (and thus the magnetometer 12) in a fixed position outside the separator chamber.

The magnetic field guide 18 is held between the separation chamber 110 and the magnetometer 12. In this particular embodiment, the magnetic field guide is substantially circular in shape with four radial cuts. It is known that in some cases, holes or cutouts improve the performance of the magnetic field guide 18, in terms of the variation in magnetic field uniformity, at the point where the magnetometer 12 is attached, for a series of different magnets. In other embodiments, the magnetic field guides may be different shapes.

In this embodiment, power for the electronics is provided by a pair of batteries 20 mounted on either side of the PCB 14. An input means is provided in the form of a pair of buttons 22 surface mounted to the PCB 14 for receiving an indication that the filter is empty and for resetting the device (i.e. clearing any stored values in memory). In this embodiment, the output device is provided in the form of an LED 24.

A snap-over cover 26 is provided to enclose and protect the electronics, including an aperture or suitable interface for operating the button 22, and for viewing the LED 24.

Referring now to fig. 2, a graph shows the results of an experiment to determine the optimal placement of a magnetometer on a particular model of magnetic filter. Each line on the graph represents a different level of magnetite dose. The X-axis is the vertical position of the magnetometer in mm and the Y-axis is the output of the magnetometer. Note that the vertical position on the X-axis is measured from a zero point located 12mm below the edge of the cylinder.

The magnetic elements in the filters of the present application are of a typical type formed from a stack of substantially cylindrical blanks having north and south poles nominally located in opposite planes, the magnets in the stack being treated as facing one another between like-named poles.

Clearly, a magnetometer located in the area of the indication X will provide a more useful output than, for example, a magnetometer located in the area of the indication Y. In region X, there is a relatively large and increasing output from the magnetometer, which is an increasing magnetite dose level. Thus, for this particular magnetic filter, the magnetometer should be positioned and held at a distance of about 23mm from zero, for example about 35mm from the underside of the can rim. For other types of magnetic filters, such as cartridges having different diameters/lengths, different sizes, locations and characteristics of the magnets, the optimal distance of the pole pieces within the magnets will be different, but can be measured as described for a particular type of filter.

Fig. 3 shows the output of the magnetometer at this position. The X-axis is the amount of magnetite added to the system and the Y-axis is the output of the magnetometer. It is clear that there is a clear relationship between these two variables. The dose of magnetite greater than the capture capacity of the magnetic filter corresponds to the noise region (region Z) on the right side of the graph. If magnetite is added continuously after the filter is fully loaded, a significant amount of magnetite remains suspended in the water of the system as it passes through the filter, which can affect magnetometer readings in an unpredictable manner. However, it is clear that by setting appropriate thresholds and/or keeping a record of previous readings, the apparatus will be able to identify when the filter is full and ignore abnormal readings caused by suspended magnetite once the filter stops capturing.

In this embodiment, the magnetometer is about 23mm from zero and the magnetometer readings increase as the magnet accumulates on the magnetic element. Thus, in this embodiment, a notification will be sent when the magnetometer reading exceeds a preset threshold. However, in other embodiments, where the magnetometer is located in a different location, or where the filter includes a different type of magnetic element, the magnetometer readings may decrease as magnetite accumulates. In such an embodiment, the notification would be issued when the magnetometer falls below the preset threshold, rather than when the magnetometer is above the preset threshold.

Fig. 4 is a computer simulation of the magnetic flux from around the magnetic element 120 in a typical magnetic filter in the presence of the magnetic field guide 18. In the indicated region B, the lighter areas of the graph represent greater flux densities, which are particularly evident where adjacent like-named magnetic blanks meet. The magnetic field guides 18 "pull" the magnetic field lines to a single known region and this is known to result in a more predictable correlation between magnetic field strength and magnetite level for a range of magnetic elements, including those having magnetic axes that are significantly skewed from their centers.

When the filter is full and needs cleaning, the monitoring device will alert the resident, the heating engineer, etc. This ensures that the filter continues to effectively protect the boiler by continuously removing magnetite, and does not need to be serviced at unnecessarily frequent intervals.

Referring now to FIG. 5, an alternative embodiment of a measuring device 10 'is shown mounted on a magnetic filter 100'. The measuring device 10 'differs from the monitoring device 10 in that the measuring device 10' is not "always on". Therefore, it does not continuously monitor the level of magnetite in the filter, but instead relies on a "push to test" system. A single momentary push button 28' is provided and when depressed, turns on the measuring device. As described above, the amount of magnetite was measured using a magnetometer, with the output displayed on a series of four LEDs 24'. The LEDs indicate full 25%, full 50%, full 75%, and full 100%. For example, when the filter is 50% full, 25% and 50% of the LEDs will be lit, while when the filter is 100% full, all of the LEDs 24' will be lit.

Another LED25' lights up in a "low battery" state to prompt the user to replace the battery in the device.

When the button 28' is not pressed, all of the LEDs 24', 25' are off and no power is drawn from the battery.

As is apparent from the drawings, the measuring device 10' has a different physical layout and structure than the monitoring device 10. However, both devices include the same necessary components, and both devices generally operate in the same manner, except that the measuring device 10 'only operates when the button 28' is pressed. It is envisaged that the monitoring means is provided in a housing similar to that shown in figure 5, possibly including communication means.

Claims

1. A method of measuring the amount of magnetite captured by a magnetic filter in a central heating system,

the magnetic filter comprises a separation chamber, an inlet of the chamber, an outlet of the chamber, and a magnetic element disposed within the separation chamber for attracting magnetic particles and removing the magnetic particles from the system water as the system water flows through the separation chamber; the magnetic element comprising a stack of substantially cylindrical magnetic blanks having north and south poles nominally located in opposite planes of the blanks, the blanks being disposed with like-name poles facing each other, each blank being disposed on a central longitudinal axis of the magnetic filter,

the method comprises the following steps:

positioning a magnetometer on the housing at a fixed point relative to the magnetic element, said fixed point being a predetermined distance from the filter tip;

reading the output of the magnetometer; and

if the output from the magnetometer is above or below a preset threshold, an indication is given on the output means.

2. The method of claim 1, wherein the output of the magnetometer is monitored and an indication is issued whenever the output from the magnetometer is above or below the preset threshold.

3. A method according to claim 1 or 2, wherein the magnetometer is arranged at a fixed point outside the chamber.

4. A method of measuring the amount of magnetite captured by a magnetic filter in a central heating system according to claim 1 or 2, wherein the output of the magnetometer is directly proportional to the magnetic field strength, with a fixed but uncalibrated offset.

5. The method of claim 4, comprising the steps of: measuring the output of the magnetometer when the filter is free of magnetite; and calculating a preset threshold value by adding a fixed offset to the measurement value.

6. A method according to claim 1 or 2, wherein a magnetic field guide is provided for guiding magnetic field lines to the area where the magnetometer is located.

7. The method of claim 6, wherein the magnetic field guide is disposed between the magnetometer and the magnetic element.

8. The method of claim 6, wherein the magnetic field guide is made of metal.

9. The method of claim 8, wherein the magnetic field guide is made of stainless steel.

10. The method of claim 9, wherein the magnetic field guide is made of grade 430 stainless steel.

11. The method according to claim 1 or 2, comprising the steps of: sampling and recording magnetometer readings; and determining the rate of change from the recorded readings.

12. The method of claim 11, wherein the rate of change is recorded at intervals.

13. The method of claim 12, wherein if the rate of change increases, an indication is issued on the output device.

14. A method according to claim 1 or 2, wherein a pressure sensing device is provided for sensing the static pressure in the separation chamber.

15. The method of claim 14, comprising the steps of: static pressure measurements were sampled and recorded.

16. A method according to claim 15 when dependent on claim 12, wherein the emission of the indication on the output device is dependent on magnetite growth data and pressure sensing data.

17. A magnetite level measuring device for a magnetic filter in a central heating system,

the magnetic filter comprising a separation chamber, an inlet to the chamber, an outlet to the chamber, and a magnetic element disposed within the chamber for attracting magnetic particles and removing them from the system water as it flows through the chamber, the magnetic element comprising a stack of substantially cylindrical magnetic blanks having north and south poles nominally located on opposite planes of the blanks, the blanks being disposed with like-name poles facing each other, each blank being disposed on a central longitudinal axis of the magnetic filter,

the measuring device includes:

a housing for placement at a fixed point relative to the magnetic element, the housing being fixed to the chamber and held in a fixed position relative to the chamber, adjacent the exterior of the chamber;

a magnetometer mounted on said housing, the output of said magnetometer being directly proportional to the magnetic field strength, having a fixed but uncalibrated offset;

an input device for receiving a notification that the filter is free of magnetite; and

an output device adapted to issue a notification when the output from the magnetometer exceeds or falls below a threshold, the threshold being calculated in response to the notification on the input device by adding a fixed offset to the output of the magnetometer at the time the notification is received.

18. A measurement device according to claim 17 adapted to monitor the output of the magnetometer and to issue said notification once the output from the magnetometer is above or below a preset threshold.

19. The measurement device of claim 17, wherein the magnetic field guide is mounted on the housing.

20. A measurement device according to claim 19, wherein the magnetic field guide is disposed between the magnetometer and the magnetic element when the housing is mounted to the separation chamber.

21. The measurement device of claim 20, wherein the magnetic field guide is made of metal.

22. The measurement device of claim 21, wherein the magnetic field guide is made of stainless steel.

23. The measurement device of claim 22, wherein the magnetic field guide is made of grade 430 stainless steel.

24. A measurement device as claimed in claim 23, comprising storage means for storing a plurality of magnetometer readings.

25. A measurement device according to claim 24 adapted to periodically sample readings from the magnetometer and record the readings onto a storage device.

26. A measurement device as claimed in claim 25, adapted to calculate the rate of change of the recorded magnetometer readings over time.

27. A measurement device as claimed in claim 26 adapted to record the rate of change at intervals.

28. A measuring apparatus according to claim 27, adapted to issue a notification on the output means if the rate of change increases.

29. A measurement device as claimed in any of claims 17 to 28, comprising pressure sensing means for sensing static pressure within the separation chamber of the filter.

30. A magnetite level measurement apparatus for a magnetic filter in a central heating system according to claim 29 adapted to periodically sample and record static pressure measurements.

31. A measurement device according to claim 30 adapted to indicate at the output device based on a combination of magnetite growth data and pressure sensor data.